Resin pellets requiring a low content of foreign substances have been known. For example, polycarbonate-based resin pellets with a low content of foreign substances or cyclohexane-based resin pellets with a low content of foreign substances used as a material for a substrate of an optical disk require reductions in the content of metallic particles derived from such a facility as a silo or pipe, the content of fine resin powder generated in the pelletizing processes; forming resin pellets through cutting resin strands extruded from an extruder, the content of small resin fragments generated through rubbed against inner wall of pipes, valves or silos when the pellet resins are pneumatically conveyed, etc. The reason is that, in an optical disk having an optical disk substrate formed of the resin pellets, the foreign substances inhibit the transmission of laser and increase errors when information recorded in the optical disk is read with the laser or when the information is written in the optical disk. Especially, in recent years, requirements for the reduction of contained foreign substances have been increasingly severe along with an increase in a storage capacity (recording density) in the optical disks. A removing device (see, e.g., Patent Document 1) which removes the foreign substances contained in the resin pellets, and a material for use in optical disk substrates which generates a small amount of resin fine powder (see, e.g., Patent Document 2) are also proposed.
Patent Document 1: Japanese Patent Application Laid-Open No. 6-270145; and
Patent Document 2: Japanese Patent Application Laid-Open No. 11-342510.
Along with an increase in the global demand for optical disk products, the amount of production of pellets with a low content of foreign substances which are typified by a polycarbonate-based resin or a cyclohexane-based resin for use in the optical disk substrate and so on, has been increasing, and its production facilities have been increasing in size. For example, the silo in which the resin pellets are stored has also been increasing in size. In such a silo, however, the first portion of resin pellets discharged from the silo and the last portion of resin pellets discharged from the silo has a higher content of foreign substances, as compared with the middle portion.
FIG. 6 is a schematic sectional view of a resin pellet store device according to a conventional example, FIG. 7 is a graph showing the content of the foreign substances in the resin pellets discharged from the resin pellet store device according to the conventional example, and FIG. 8 is an explanatory view showing in time series a behavior that the resin pellets are discharged from the resin pellet store device according to the conventional example.
As shown in FIG. 6, a general silo 100 for use as a resin pellet store device includes a filling inlet 101 through which the resin pellets are filled; a barrel part 102 in which the resin pellets are stored; a cone part 103 constituted by tapering a lower part of the silo 100 toward the center thereof; a discharge outlet 104 locating at the lowermost part of the silo 100 through which the resin pellets are discharged; a sampling outlet 105 through which the resin pellets stored above the lowermost part of the silo 100 are taken as a sample; and level meters 106.
FIG. 7 shows the change of the content of foreign substances measured while discharging the filled resin pellets from the silo 100 until the silo is emptied. The content of the foreign substances tends to increase in the first portion and last portion of resin pellets discharged from the silo 100.
This may be caused due to the influence of fine resin powder or small resin fragments sticking to the inner wall of the silo 100 by static electricity. For example, as shown in FIG. 8, the resin pellets in the silo 100 firstly fall down from the center thereof and are discharged therefrom as if sand fell in a so-called sand clock. Lastly, the outer peripheral pellets fall down and are discharged therefrom. Therefore, the last portion of resin pellets discharged tends to contain a large amount of fine resin powder or small resin fragment sticking to the inner wall of the silo 100. Especially, the last portion of resin pellets discharged fall down while rubbing against the inner wall of the cone part 103. In consequence, a large amount of foreign substances such as the metallic particles, the fine resin powder and the small resin fragments accumulated in the lowermost part may be contained in the last portion of pellets discharged from the silo 100.
In large-sized production facilities, there may be another packing facility such as a receptacle or container on the downstream side of the silo. In such a case, there may be a possibility that the downstream facility may be contaminated with the foreign substances contained in the last portion of resin pellets discharged from the silo. As a result, the first portion of resin pellets discharged from the silo at the next lot packing may also be contaminated.
Therefore, the last portion of resin pellets discharged should be separately treated as OFF-SPEC products through other routes.
The amount of the pellets to be treated as OFF-SPEC products is an amount which exceeds a specified batch quantity (a capacity of a transport receptacle such as a flexible container or a lorry) at a boundary between the lots, depending on the size of the silo, the specifications of the resin pellets, the definition of the lot and so on. These pellets are treated as the so-called YAMAKAKE (the last portion of resin pellets of which the quantity is less than the specified batch quantity), without being mixed with the pellets of another (next) lot.
However, it is actually difficult to sort out ON-SPEC products and OFF-SPEC products. Therefore, there is a possibility that the resin pellets to be treated as the OFF-SPEC products are mixed with the ON-SPEC products, which might increase the content of the foreign substances in ON-SPEC products. In the case where the last portion of resin pellets discharged are treated in a route different from that for the ON-SPEC products, it is necessary to switch the routes, leading to a problem of laborious facility management.
In order to prevent the last portion of resin pellets discharged from becoming OFF-SPEC products, a technique of homogenizing resin pellets in the silo by using a silo homogenizing device referred to as an after-blender may be used. However, the after-blender mixes the fine resin powder and the small resin fragments temporarily sticking to the inner wall of the silo by the static electricity with resin pellets again, which might increase the content of those foreign substances. Therefore, for the resin pellets requiring a low content of foreign substances, which are used as the low material of optical disk substrates and so on, the after-blender can not be considered to be a suitable solution method.
In addition, Patent Document 1 discloses a removing device for removing fine resin powder from the resin pellets. This removing device is installed in resin pellet supply route between the silo in which the resin pellets are stored and a transport receptacle such as the lorry. Fine resin powder can be actively removed from the resin pellets by blowing an ionized air into the removing device. Then, removed fine resin powder is discharged together with air through the exhaust outlet. In Patent Document 1, however, there is not any detailed description concerning the silo.
Moreover, Patent Document 2 discloses a material for use in the optical disk substrate in which the generation of fine powder is reduced. In this material, the content of the resin fine powder having 1.0 mm or less of a diameter is set at 250 ppm or less, further preferably 150 ppm or less. However, in the case of the resin pellets having a low content of foreign substances, which will be used for DVDs such as DVD-Rs and DVD-RAMs, the above can not satisfy the requirements of disk manufacturers, unless the content of fine resin powder may be reduced down to less than 50 ppm, preferably to less than 30 ppm.
Furthermore, in Patent Document 2, pelletizing conditions of strands extruded from an extruder are described. Accordingly, Patent Document 2 does not include any specific description concerning the silo in which the resin pellets are stored.
The present invention has been developed in view of the above circumstances, and an object of the present invention is to provide a resin pellet store device and a cleaning method thereof in which the portion of pellets with a higher content of foreign substances to be treated as OFF-SPEC products are properly sorted out. Besides, the OFF-SPEC products are separately discharged through another route, in order to prevent ON-SPEC products having a low content of foreign substances from being contaminated in a downstream facility.